The switching power supply can set an output voltage to a desired value by controlling ON and OFF operation of the switching element. FIG. 1 illustrates an example of a circuit using as such a switching power supply.
A circuit as shown in FIG. 1 is constructed as set forth below. It should be noted that in FIG. 1, reference numerals 1 and 2 indicate an input terminal and an output terminal of high potential sides, respectively, and an input terminal and an output terminal of low potential sides are omitted, but each terminal is grounded.
A choke coil L1 and a switching transistor Q1 are connected between the input terminal 1 and the ground, a diode D1 is connected between a connection point of the choke coil L1 and the switching transistor Q1, and the output terminal 2, and a smoothing capacitor C2 is connected between the output terminal 2 and the ground. A capacitor C1 for filter is connected between the input terminal 1 and the ground. A boost chopper circuit is formed by connecting these elements.
In order to operate this boost chopper circuit, a control circuit 3 and a driving circuit 4a are arranged, wherein the driving circuit 4a is formed of a resistor R2, transistors Q2 and Q3 which are connected in series, and a resistor R1 connected between the input terminal 1 and a common connection point of each base of the transistors Q2 and Q3.
A common connection point of each emitter of the transistors Q2 and Q3 in the driving circuit 4a is connected to a base of the switching transistor Q1, and a common connection point of each base of the transistors Q2 and Q3 is connected to a pulse output terminal PO of the control circuit 3. A power supply terminal Vcc of the control circuit 3 is connected to the input terminal 1, and a terminal GND is grounded. A voltage detecting terminal FB of the control circuit 3 is connected to a mid-connection point of resistors R3 and R4, which are connected in series between the output terminal 2 and the ground.
In the circuit as shown in FIG. 1, when pulse signals are supplied from the control circuit 3, the transistors Q2 and Q3 of the driving circuit 4a are operated complementarily and drives the switching transistor Q1. For example, if the pulse signal which the control circuit 3 outputs is high in voltage value, the transistor Q2 is in an ON-state and the transistor Q3 is in an OFF-state. In this case, a forward bias is applied through the resistor R2 and the transistor Q2 to the base of the switching transistor Q1, thereby the switching transistor Q1 becomes ON.
On the contrary, if the pulse signal is low in a voltage value, the transistor Q2 is in an OFF-state and the transistor Q3 is in an ON-state. In this case, a forward bias is not applied to the base of the switching transistor Q1, and the base of the transistor Q1 is grounded through the transistor Q3, so that the switching transistor Q1 becomes OFF.
In this connection, during an ON-period, positive charges are stored in a base region and negative charges in an emitter region of the switching transistor Q1. These stored charges are important in operation. This is because the switching transistor Q1 is turned to the OFF-state by extinction of the stored charges.
When the switching transistor Q1 is turned from the ON-state to the OFF-state in FIG. 1, the transistor Q3 becomes ON and short-circuits between the base and emitter of the switching transistor Q1. At this time, a discharge path of the stored charges of the base and emitter regions of the switching transistor Q1 is formed by operation of the transistor Q3 and extinction of the stored charges is improved, so that turn-off operation of the switching transistor Q1 is speeded up.
Then, in order to disappear rapidly the stored charges of the base and emitter regions of the switching transistor Q1, it is desirable that there is neither electric resistance nor potential difference to disturb a flow of the charges on a discharge path of the charges. As is well known, however, between terminals of a transistor element, the potential difference based on a PN junction thereof is caused. In this connection, in the transistor Q3 in FIG. 1 the potential difference (a voltage V.sub.BE between a base and an emitter) is also caused. For this, in the circuit in FIG. 1 the potential difference appears, based on a voltage between the base and emitter of the transistor Q3, on the discharge path between the base and emitter of the transistor Q1, and discharge of the stored charges is disturbed.